Devices and methods relating to electrochemical biosensors

ABSTRACT

A system for testing for analytes in a sample of biological fluid includes a test strip that defines a cavity for receiving the sample. At least two sets of electrodes are adjacent the sample cavity, including one for measuring one property of the sample, and another for measuring one or more other properties of the sample, such as temperature and/or the presence or magnitude of confounding variables. The measurements are combined to yield the desired result. At least one set of working and counter electrodes each have a plurality of elongated “fingers” interdigitated with those of the other electrode in the set. The gaps between fingers can be quite small, so that the two electrode sets together can operate in a small measurement volume of sample. Additional electrodes can be included that measure the presence or sufficiency of the sample, and additional traces on the strip can act as configuration identifiers.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/872,008, filed Jun. 18, 2004, which claims the benefit of U.S.Provisional Application No. 60/480,243, filed Jun. 20, 2003, titled“DEVICES AND METHODS RELATING TO ELECTROCHEMICAL BIOSENSORS.” Thisapplication is also related to applications titled SYSTEM AND METHOD FORANALYTE MEASUREMENT USING AC EXCITATION (U.S. application Ser. No.10/688,343, “AC Excitation application” herein), METHOD OF MAKING ABIOSENSOR (case number RDID-9958-CIP-US, “Biosensor application”herein), and DEVICES AND METHODS RELATING TO ANALYTE SENSORS (U.S.Provisional Application No. 60/480,397, “Analyte Sensors application”herein), which were all filed on Jun. 20, 2003, and to U.S. patentapplication Ser. No. 10/264,891 (entitled “ELECTRODES, METHODS,APPARATUSES COMPRISING MICRO-ELECTRODE ARRAYS”, filed Oct. 4, 2002),which are all incorporated herein by reference in their entireties.

BACKGROUND

The present invention relates to devices, systems, and methods formeasuring analytes from biological samples, such as from a sample ofbodily fluid. More specifically, the present invention relates tobiosensors and methods for testing an analyte using certain electricalresponse characteristics.

Measuring the concentration of substances, particularly in the presenceof other, confounding substances (“interferents”), is important in manyfields, and especially in medical diagnosis and disease management. Forexample, the measurement of glucose in bodily fluids, such as blood, iscrucial to the effective treatment of diabetes.

Multiple methods are known for measuring the concentration of analytessuch as glucose in a blood sample. Such methods typically fall into oneof two categories: optical methods and electrochemical methods. Opticalmethods generally involve absorbance, reflectance or laser spectroscopyto observe the spectrum shift in the fluid caused by the concentrationof the analytes, typically in conjunction with a reagent that produces aknown color when combined with the analyte. Electrochemical methodsgenerally rely upon the correlation between a charge-transfer orcharge-movement property of the blood sample (e.g., current, interfacialpotential, impedance, conductance, and the like) and the concentrationof the analyte, typically in conjunction with a reagent that produces ormodifies charge-carriers when combined with the analyte. See, forexample, U.S. Pat. Nos. 4,919,770 to Preidel, et al., and 6,054,039 toShieh, which are incorporated by reference herein in their entireties.

An important limitation of electrochemical methods of measuring theconcentration of a chemical in blood is the effect of confoundingvariables on the impedance of a blood sample. For example, the geometryof the blood sample must correspond closely to that upon which theimpedance-to-concentration mapping function is based.

The geometry of the blood sample is typically controlled by asample-receiving portion of the testing apparatus. In the case of bloodglucose meters, for example, the blood sample is typically placed onto adisposable test strip that plugs into the meter. The test strip may havea sample chamber to define the geometry of the sample. Alternatively,the effects of sample geometry may be limited by assuring an effectivelyinfinite sample size. For example, the electrodes used for measuring theanalyte may be spaced closely enough so that a drop of blood on the teststrip extends substantially beyond the electrodes in all directions.Regardless of the strategy used to control sample geometry, typicallyone or more dose sufficiency electrodes are used to assure that there isa sufficient amount of sample to assure an accurate test result.

Other examples of limitations to the accuracy of blood glucosemeasurements include variations in blood chemistry (other than theanalyte of interest being measured). For example, variations inhematocrit (concentration of red blood cells) or in the concentration ofother chemicals, constituents or formed elements in the blood, mayaffect the measurement. Variation in the temperature of blood samples isyet another example of a confounding variable in measuring bloodchemistry.

Thus, a system and method are needed that accurately measure bloodglucose, even in the presence of confounding variables, includingvariations in temperature, hematocrit, and the concentrations of otherchemicals in the blood. A system and method are likewise needed thataccurately measure an analyte in a fluid. It is an object of the presentinvention to provide such a system and method.

Many approaches have been employed to attenuate or mitigate theinfluence of one or more sources of interference, or to otherwisecompensate for or correct a measured value. Often multiple designsolutions are employed to adequately compensate for the sensitivitiesassociated with the chosen measurement method.

Well known design solutions involve perm-selective and/or size-selectivemembranes, filters or coatings. Such design solutions suffer fromincremental costs of goods, additive manufacturing process steps furtherexacerbating manufacturing cost, complexity, and speed of manufacture.Systems (disposable test strips and instruments) employing these methodstake the general approach of overcoming the problem within the scope ofthe test strip design.

Another general approach involves the use of sophisticated excitationand signal processing methods coupled with co-optimized algorithms.Simpler, less complex, test strip architectures and manufacturingprocesses may be realized; however, instrumentation costs, memory andprocessor requirements, associated complex coding, and calibratedmanufacturing techniques are required. Systems employing this techniquetake the general approach of overcoming the problem within the scope ofthe instrumentation.

A more recent approach involves neither the strip nor instrumentation,per se, but rather exploits the measurement methodology. An example ofthis is the use of a coulometric method to attenuate the influence ofhematocrit and temperature.

It is also well known to those skilled in the art that all of the aboveapproaches are further supported by the initial design of reagentsystems. In the detection of glucose, for example, this may involve theuse of selective redox mediators and enzymes to overcome the detrimentalinfluence of redox-active species or the presence of other sugars.

It is an object of the invention to provide a simpler, less costlymethod for attenuating the influence of interferents, in a manner thatdoes not suffer the demerits associated with the general approachescurrently in wide use.

SUMMARY Two Pairs Generally.

In one aspect, the present invention involves the provision of two pairsof electrodes, which allow for the use of two measurements to correct orcompensate the analyte measurement for interferents. In one embodimentfor example, a pair of electrodes defines a first measurement zone,while a second pair defines a second measurement zone. The pairs areroughly coplanar, and within a pair of electrodes each has a lengthsubstantially parallel to the length of the other. At least one of theelectrodes in the first pair of electrodes comprises at least twoelongated, rectangular conductive elements, which are interdigitatedwith the conductive element(s) of the other electrode in the pair. Eachelement for an electrode is conductively connected to the same contactfor electrical communication with a driver and/or meter. The sampleestablishes electrical contact with both pairs after dosing.

Several variations of the foregoing are contemplated. For example, inone approach a reagent or a plurality of reagents can be selectivelydeployed onto at least one of the at least two pairs of electrodesresiding in a sample chamber. Both pairs are coated with a firstreagent. Optionally, one of the two pairs is coated with a firstreagent, and the second pair is coated with the same reagent but lackingeither enzyme or mediator. Alternatively, one of the two pairs is coatedwith a first reagent and the other pair is coated with a second reagent.In another embodiment, one of at least two pairs is coated with areagent and the other pair lacks a reagent coating, with the downstreampair preferably having the reagent coating. In a variation of thisembodiment, the other of the pairs is covered with a coating that isperm-selective, size-selective, or otherwise affects the electroderesponse in the presence of one or more analytes and/or interferents.

In further aspects, dose detection and dose sufficiency electrodes areincluded. For example, a third electrode system may be included that islocated further from the edge than the first two electrode pairs, i.e.is downstream of the entering sample fluid, and is operable to detectwhen there is sufficient sample fluid to conduct an accurate test. Thisthird electrode system may comprise a single electrode element or aplurality of elements. In single-element embodiments, the elementfunctions in combination with one or more of the other electrodes totest for sample sufficiency. Alternatively, the dose sufficiencyelectrode system may comprise a pair of electrode elements thatcooperate with one another to evidence sample sufficiency. A comparableelectrode system may similarly be employed to detect when a sample fluidhas been applied to the biosensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a testing strip according to oneembodiment of the present invention.

FIG. 2 is an exploded view of selected layers of the test strip of FIG.1.

FIG. 3 is a cutaway plan view of the electrode portion of the strip ofFIG. 1.

FIGS. 4-15 are exploded views of alternative test strips according tothe present invention.

DESCRIPTION

For the purpose of promoting an understanding of the principles of thepresent invention, reference will now be made to the embodimentsillustrated in the drawings and specific language will be used todescribe the same. It will, nevertheless, be understood that nolimitation of the scope of the invention is thereby intended; anyalterations and further modifications of the described or illustratedembodiments, and any further applications of the principles of theinvention as illustrated therein are contemplated as would normallyoccur to one skilled in the art to which the invention relates.

Introduction

Generally, the test strips of the present invention provide for testingof an analyte in a bodily or other fluid using multiple electrode arraysthat perform different functions or have different response functionswith the sample. One particular embodiment involves the combination ofmacro-electrodes and micro-electrodes that operate in respective pairs,but contribute information for a final determination of the analyteconcentration, such as by having the information obtained from oneelectrode pair being used to compensate or correct the results obtainedfrom the other electrode pair, or by combining the responses of theelectrode pairs in a predetermined fashion.

These electrode arrays may also be combined in a wide variety of otherways to accomplish multiple related functions, including analyteconcentration, detection of hematocrit, determination of correctionfactors, as well as sample sufficiency and dose detection, all on asingle strip and in an extremely small space. Alternatively, by usingmultiple arrays with different sensitivities to interferents, one mayexploit the two measurements to provide a more accurate result, as wouldnormally occur to one skilled in the art.

In various embodiments, different electrochemical excitation techniques(for example, DC, AC phase, AC amplitude, or combined DC/AC) are appliedto these different electrode arrays to achieve the desired goals.Examples of such techniques are well known in the art, and are furtherexemplified in the AC Excitation application, which was incorporatedabove by reference.

Another exemplary technique compensates for variation of diffusioncoefficients of the electrochemically active species being tested.Faradaic current in soluble reagents at an electrode surface occurs dueto the physical diffusion of these species and the value of thediffusion coefficient influences the measured response. Commercialsystems are often calibrated and built such that the nominal sensorresponse (faradaic current) to a given amount of glucose is repeatableif the diffusion coefficients remain fixed. Unfortunately both thetemperature and hematocrit (HCT) of each individual sample alter theeffective diffusion coefficient of the electroactive species beingmeasured. If these factors are not considered, the glucose measurementcan be in error for any temperature or hematocrit value differing fromthose used in the calibration of the system.

In this exemplary technique, the system determines the faradaic responseof an electrochemical sensor due to an analyte of interest, and providesan estimate of the actual, effective diffusion coefficient of thespecies undergoing the redox reaction at the electrode surface. Inparticular, the system compensates for diffusion coefficient variationby using two electrode systems (preferably of different types) exposedto the same reagent-sample mixture. Soluble, electroactive species, suchas the redox mediators commonly employed in glucose biosensors, diffuseto a planar, macro-electrode yielding a current response to a potentialstep according to the Cottrell equation (1).

$\begin{matrix}{i_{p} = {{nFA}_{p}C\sqrt{\frac{D}{\pi \; t}}\mspace{14mu} {so}\mspace{14mu} {that}}} & \left( {1a} \right) \\{{\lim\limits_{t\rightarrow{t{(\infty)}}}i_{p}} = 0} & \left( {1b} \right)\end{matrix}$

where n is the number of electrons involved in the electron transfer, Fis the Faraday constant (96,485.3 C/equivalent), A_(p) is the area ofthe macro-electrodes in contact with the solution, C is theconcentration of the analyte in the sample, D is the effective diffusioncoefficient of the species, and i_(p) is the current response at themacro-electrode.

It will similarly be understood by those of skill in the art that theresponse of these same species to the same potential step at amicro-electrode would yield a current response characterized by equation(2).

$\begin{matrix}{i_{s} = {{{nFA}_{s}C\sqrt{\frac{D}{\pi \; t}}}\mspace{14mu} + {\frac{{vnFA}_{s}{CD}}{r_{o}}\mspace{14mu} {so}\mspace{14mu} {that}}}} & \left( {2a} \right) \\{{\lim\limits_{t\rightarrow{t{(\infty)}}}i_{s}} = \frac{{nFA}_{s}{CD}}{r_{o}}} & \left( {2b} \right)\end{matrix}$

where A_(s) is the area of the micro-electrode, ν is an electrodeshape-dependent accessibility factor and i_(s) is the current responseat the micro-electrode at the micro-electrode. In equations (1b) and(2b), t(∞) means a time sufficiently long that the condition of“semi-infinite” or “steady-state” diffusion, respectively, can beestablished at the electrodes in question.

One embodiment would apply the same potential between (a) the planar,macro electrode and a counter/reference electrode, and between (b) themicro electrode(s) and counter/reference electrode. The time-dependentcurrent response would then be measured at several time-points followingpotential application at both macro and microelectrodes. An analysis of

$i_{p} = {f\left( \frac{1}{\sqrt{t}} \right)}$

would produce slope_(p), as in equation (3), while the same analysis of

$i_{s} = {f\left( \frac{1}{\sqrt{t}} \right)}$

would yield an intercept_(s) as shown in equation (4).

$\begin{matrix}{{slope}_{p} = {{nFA}_{p}C\sqrt{\frac{D}{\pi}}}} & (3) \\{{intercept}_{s} = \frac{{nFA}_{s}{CD}}{r_{o}}} & (4)\end{matrix}$

Given that in this invention both i_(p) and i_(s) are derived from thesame reaction and sample, it is possible to calculate an apparentdiffusion coefficient for the electrochemically reacting species in thedevice, independent of the concentration of the species according toequation (5), where the areas of the two electrode types, A_(s) andA_(p), as well as the radius of the micro-electrode(s), r_(o), areknown. For example, a spherical microelectrode yields:

$\begin{matrix}{\frac{{intercept}_{s}}{{slope}_{p}} = \frac{A_{s}\sqrt{\pi \; D}}{r_{o}A_{p}}} & (5)\end{matrix}$

Once D is estimated, it can be applied in a number of different ways toprovide a correction for the measured concentration, C, of theelectrochemical species. Some embodiments simply use the estimated valueof D in equation (3) to calculate C. Such a determination of C is lesssubject to uncompensated variation in D as is common in amperometricsensors whose current response is largely described by equation (1). Itis also noteworthy that the correction is independent of the cause ofvariation in D (e.g., temperature, hematocrit, viscosity change,etc.)—the correction is provided by the different functional dependenceof the two electrode pairs on the chemical properties of the sample.

In each of the strips illustrated herein, an electrode array is used tomeasure an analyte, such as glucose, in a sample. When the samplereaches the array, it combines with reagent that is placed adjacent tothe array to provide certain properties of electrical impedance in thepresence of a certain electrical signal, as is understood in the art,which impedance is used as a first datum. Another array, either upstreamor downstream from the first array, but preferably not covered by areagent, is used to provide another electrical stimulus to the sample,and the electrical response at the array is used as a second datumaffected in a known way by an interferent, such as hematocrittemperature, or the like. This two data are combined to yield acorrected analyte concentration value. The two arrays can be used at thesame time to analyze a single sample in a common volume of very smalldimensions.

General Information

System

The present invention relates to a system that is useful for assessingan analyte in a sample fluid. The system includes devices and methodsfor evaluating the sample fluid for the target analyte. The evaluationmay range from detecting the presence of the analyte to determining theconcentration of the analyte. The analyte and the sample fluid may beany for which the test system is appropriate. For purposes ofexplanation only, a preferred embodiment is described in which theanalyte is glucose and the sample fluid is blood or interstitial fluid.However, the present invention clearly is not so limited in scope.

Sensor

One component of the system is an electrochemical sensor including asample-receiving chamber for the sample fluid, and a reagent forproducing an electrochemical signal in the presence of the test analyte.The sensor preferably comprises a disposable test strip, particularlyone having a laminar construction providing an edge opening to asample-receiving chamber. The reagent is disposed within thesample-receiving chamber in position to provide the electrochemicalsignal to a working electrode also positioned within the chamber. Inappropriate circumstances, such as for glucose detection, the reagentmay contain an enzyme and optionally a mediator.

Meter

The sensor is used in combination with a meter for determination of thepresence and/or concentration of the analyte in the sample fluid. Themeter conventionally includes a connection with the electrodes of thesensor and circuitry to evaluate the electrochemical signalcorresponding to the concentration of the analyte. The meter may alsoinclude means for determining that the sample fluid has been received bythe sensor, and that the amount of sample fluid is sufficient fortesting. The meter typically will store and display the results of theanalysis, or may alternatively provide the data to a separate device.

Analyte—Characteristic

The system can provide either a qualitative or quantitative indicationfor the analyte. In one embodiment, the system indicates simply thepresence of the analyte in the sample fluid. The system may also providea reading of the quantity or concentration of the analyte in the samplefluid. In a preferred embodiment, it is a feature of the presentinvention that a highly accurate and precise reading of the analyteconcentration is obtained.

Analyte—Type

The system is useful for the determination of a wide variety ofanalytes. The test strip, for example, is readily adapted for use withany suitable chemistry that can be used to assess the presence of theanalyte. Most preferably, the system is configured and used for thetesting of an analyte in a biological fluid. Such analytes may include,for example, glucose, lactate, urate, ketones, etc. Commensuratemodifications to the system will be apparent to those skilled in theart. For purposes of explanation, and in a particularly preferredembodiment, the system is described with respect to the detection ofglucose in a biological fluid.

Interferents

Test methodologies may be variously affected by the presence ofinterferents in the sample fluid. For example, the testing for glucosein a blood sample may be impacted by such factors as bilirubin,hematocrit, uric acid, ascorbate, acetaminophen, galactose, maltose, andlipids. The present system is adaptable to minimize or eliminate theadverse effects of interferents that may also be present in the samplefluid. These effects may be addressed by appropriate selection of testmaterials and parameters, such as by the selection of chemistries thatare known to be impacted less, or not at all, by possible interferents.They may also be addressed by selection of two or more reagents thathave differential sensitivities to the interferent, but substantiallythe same sensitivity to the analyte of interest. As is known in the art,other steps may also be taken to deal with possible interferent effects,such as the use of coatings or films that prevent the interferent fromentering the test zone. In addition, modifications to the electrodeconfigurations or interrogation methods can be used to minimize theeffect of interferents.

Fluid Type

The system is useful with a wide variety of sample fluids, and ispreferably used for the detection of analytes in a biological fluid. Inthis context, the term “biological fluid” includes any body fluid inwhich the analyte can be measured, for example, interstitial fluid,dermal fluid, sweat, tears, urine, amniotic fluid, spinal fluid andblood. The term “blood” in the context of the invention includes wholeblood and its cell-free components, namely plasma and serum. Inaddition, the system is useful in connection with reference fluids thatare used in conventional fashion to verify the integrity of the systemfor testing.

In a preferred embodiment, the system is employed for the testing ofglucose. The sample fluid in this instance may specifically include, forexample, fresh capillary blood obtained from the finger tip or approvedalternate sites (e.g., forearm, palm, upper arm, calf and thigh), freshvenous blood, and control solutions supplied with or for the system.

The fluid may be acquired and delivered to the test strip in anyfashion. For example, a blood sample may be obtained in conventionalfashion by incising the skin, such as with a lancet, and then contactingthe test strip with fluid that appears at the skin surface. It is anaspect of the present invention that the test strip is useful with verysmall fluid samples. It is therefore a desirable feature of theinvention that only a slight incising of the skin is necessary toproduce the volume of fluid required for the test, and the pain andother concerns with such method can be minimized or eliminated.

Electrodes

Electrode Type

The invention relates to an “electrochemical sensor”, which is a deviceconfigured to detect the presence and/or measure the concentration of ananalyte by way of electrochemical oxidation and reduction reactionswithin the sensor, and/or development of movement of charged layerswithin the solution. These reactions are transduced to an electricalsignal that can be correlated to an amount or concentration of theanalyte. The test strip therefore includes an electrode systemcomprising at least a working electrode and a counter electrode withinthe sample receiving chamber. The sample receiving chamber is configuredsuch that sample fluid entering the chamber is placed in electrolyticcontact with both the working electrode and the counter electrode. Thisallows electrical current to flow between the electrodes to effect theelectrooxidation or electroreduction of the analyte or its products.

In the context of the present invention, a “working electrode” is anelectrode at which analyte or product is electrooxidized orelectroreduced with or without the agency of a redox mediator. The term“counter electrode” refers herein to an electrode that is paired withthe working electrode and through which passes an electrochemicalcurrent equal in magnitude and opposite in sign to the current passedthrough the working electrode. The term “counter electrode” is meant toinclude counter electrodes that also function as reference electrodes(i.e., a counter/reference or auxiliary electrode).

Electrode Material

The working and counter electrodes, and the remaining portions of theelectrode system, may be formed from a variety of materials, as known inthe art. The electrodes should have a relatively low electricalresistance and should be electrochemically inert over the operatingrange of the test strip. Suitable conductors for the working electrodeinclude gold, palladium, platinum, carbon, titanium, ruthenium dioxide,iridium, and indium tin oxide, as well as others, such as the conductorsdisclosed in the Analyte Sensors application, which was incorporatedabove by reference. The counter electrode may be made of the same ordifferent materials. In a preferred embodiment, both of the electrodesare gold electrodes.

Electrode Application

The electrode systems utilized by the present invention may be appliedto the base substrate in any fashion that yields electrodes of adequateconductivity and integrity. Exemplary processes are well known in theart, and include, for example, sputtering, printing, etc. In a preferredembodiment, the electrodes and other conductive components are providedby coating a base substrate and then removing selected portions of thecoating to yield the components. A preferred removal method is laserablation, and more preferably broad-field laser ablation, as disclosedin the “Method of Making a Biosensor” application, which wasincorporated above by reference, and further relevant discussion isfound in U.S. patent application Ser. Nos. 09/866,030 (entitled“Biosensors with Laser Ablation Electrodes with a Continuous CoverlayChannel,” filed May 25, 2001) and 09/411,940 (entitled “Laser DefinedFeatures for Patterned Laminates and Electrode,” filed Oct. 4, 1999).Various other methods of fabrication and application are well known inthe art for providing the electrical components, and particularly theelectrode systems, described herein.

Reagent Composition

The test strip includes a chemical reagent within the sample receivingchamber for reacting with the test analyte to produce theelectrochemical signal that represents the presence of the analyte inthe sample fluid. The test chemistry is selected in respect to theanalyte to be assessed. As is well known in the art, there are numerouschemistries available for use with each of various analytes, includingbut not limited to the preferred chemistry described in patentapplication titled “Reagent Stripe for Test Strip” (attorney docketnumber 7404-566), which is being filed on even date herewith. Theselection of an appropriate chemistry is therefore well within the skillin the art, and further description herein is not required in order toenable one to make and use the present invention.

For purposes herein, however, a preferred embodiment is described inwhich the analyte is glucose, although it is to be understood that thescope of the invention, and of the claims, is not so limited, unlessspecifically indicated. In the case of glucose, the active components ofthe test chemistry will typically include an enzyme for glucose and aredox mediator. The enzyme oxidizes glucose in the sample, and themediator in turn reacts with the reduced enzyme. The mediator thereaftershuttles the redox equivalent of analyte product to the electrodesurface by diffusion. There the mediator is oxidized quantitatively at adefined anodic potential and the resulting current is related to theapparent glucose concentration. There are a number of reagent systemssuitable for the detection of glucose, and examples of these arecontained in the AC Excitation, Analyte Sensors, and Biosensorapplications, U.S. Pat. Nos. 5,385,846 and 5,997,817, and U.S. (Reissue)patent application Ser. No. 10/008,788 (“Electrochemical Biosensor TestStrip”), which are hereby incorporated by reference.

The glucose chemistry utilizes the redox mediator to mediate a currentbetween the working electrode and the glucose analyte, which otherwiseis not well suited for direct electrochemical reaction on an electrode.The mediator functions as an electron transfer agent that shuttleselectrons between the analyte and the electrode. A great number of redoxspecies are known and can be used as the redox mediator. In general, thepreferred redox mediators are rapidly reducible and oxidizablemolecules. Examples include ferricyanide, nitrosoaniline and derivativesthereof, and ferrocene and its derivatives.

Measurement Scheme

In one aspect of the present invention, a first pair of electrodesprovides a first measurement that is combined with a second measurementobtained with a second pair of electrodes. As previously described, aconventional test strip employs at least two pairs of electrodes (each,e.g., a working electrode and a counter electrode) to determine theanalyte concentration based upon the reaction of the analyte with areagent located on or adjacent one of the electrode pairs. A basicmeasurement of the analyte concentration is thereby obtained. However,it is often desirable to correct or compensate that measurement forother factors, such as hematocrit, temperature, the presence of otherspecies in the sample fluid, and the like. In one embodiment of thepresent invention, there is provided a biosensor and method whichemploys two pairs of electrodes, one to make the basic measurement ofthe analyte and the other to provide such correction or compensation forthe basic measurement, in some instances to yield a final measurementfigure.

The use of two pairs of electrodes may involve the use of disparateelectrode sets, in which one pair comprises macro-electrodes and theother pair comprises micro-electrodes. As used herein, the termmacro-electrode refers to an electrode whose primary effective diffusioncharacteristic is perpendicular to the surface of the electrode.Macro-electrodes are dimensioned and arranged so that the primarydiffusion characteristics are linear diffusion characteristics. The termmicro-electrode refers to electrodes exhibiting convergent,steady-state, or quasi-steady-state diffusion on the characteristic timescale of the measurement. A micro-electrode is an electrode to whichradial diffusion provides a significant alteration in the responsefunction. Micro-electrodes, for example, can be dimensioned andpositioned such that their primary impedance characteristics arecharacteristic of edge-to-edge kinetics, e.g., between the nearest edgesof the fingers. More of this functionality will be discussed withrespect to the example embodiments shown in the drawings.

One advantage of using the micro-electrodes is that these devices can beconfigured and operated to very rapidly reach a quasi-steady state ofcurrent flux at the electrodes, for example in as little as 0.50 to 3.25seconds, or even in less than one-half second. This rapid acquisition ofquasi-steady state allows for a faster and more accurate determinationof analyte concentration. This is contrasted with prior art approacheswhich have, for example, estimated or projected the result based onreadings taken before a quasi-steady state is reached.

A further advantage seen in some embodiments of the invention is thatthe quasi-steady state response to application of a DC signal is at ahigher magnitude than the quasi-steady state in many prior art systems.This improves the signal-to-noise ratio of the signal, thus enabling thesystem to provide a more accurate result.

A still further advantage seen with the interdigitated arrays ofelectrode fingers used in some forms of the present invention is thedramatically increased electrode edge length that can be achieved withina given space. Depending on the design, results can be derived in thosesystems with smaller samples, yet achieving the same quality of resultsas systems requiring larger samples.

It is noted that equations can be derived and used for the variousmicro-electrode configurations as would occur to those of ordinary skillin the art given this disclosure and the AC Excitation application,which was incorporated above by reference. It is also possible to useempirical measurements to directly determine the response function ofthe electrochemical structures present in each sensor design. It isnoted that neither an analytic description of the response functions,nor attainment of a steady-state current are necessary for improvedsystem performance.

General Description—Structure

The present invention provides electrode structures and systems that areuseful in a wide variety of biosensor devices. Described herein areexemplary test strip configurations that demonstrate the utility of thepresent invention. It will be appreciated, however, that the principlesof the present invention are equally applicable in various otherbiosensor designs. The particular compositions, sizes and othercharacteristics of the basic biosensor components are not critical andare therefore not limiting.

With reference to FIG. 1, generally, strip 210 has a first end 211 forcommunication with driving circuitry and metering circuitry (not shown),while end 218 is adapted to receive the bodily fluid in contact withelectrodes as will be discussed herein. The driving circuitry provides aknown current and/or potential through contacts 216 and monitors thecurrent and/or voltage response over a period of time. The respectivesignals travel between contacts 216 and the electrodes (shown in FIGS.2-14) via conductors 270, 272, 274, and 276. These conductors are madeof any, or a combination, of a variety of conductive materials,including for example gold or carbon, as would be understood by thoseskilled in the art.

At end 218, notched fluid guide 214 is generally rectangular, withrectangular notch 148 cut therefrom, as can be seen in FIG. 2. Fluidguide 214 lies on the substrate layer 212 (a polyimide or othermaterial, as disclosed in the “Method of Making a Biosensor”application, which was incorporated above by reference, or otherwiseknown in the art), and provides an opening 251 (see FIG. 2) for thefluid to be drawn from edge 224 toward vent 262 by capillary action.Cover layer 250 lies on top of guide layer 236 and provides an uppercontainment for the fluid path defined in part by notch 248. Thesestructures will be discussed in more detail below.

Turning now to FIG. 2, with continuing reference to certain structuresshown in FIG. 1, strip 210 includes substrate layer 212, reagent stripe264, fluid guide 214, and cover layer 218. When assembled, passageway248 is defined horizontally by inner notch surfaces 249, above by bottomsurface 258 of cover layer 218, and below by reagent stripe 264 (whichlies over electrode pair 284, but not over electrode pair 280) andelectrode region 266 on upper substrate surface 232. During a testingoperation, the fluid being tested enters passageway 248 through end 240of fluid guide 214, past edges 254 and 224 of cover layer 218 andsubstrate 212, respectively. The fluid is drawn by capillary action intopassageway 248, following a path extending away from edges 224 and 254,and toward vent 262 (see FIG. 1).

The capillary passageway provides a sample receiving chamber in whichthe measuring electrodes and associated reagent are contained, and thefluid sample containing the analyte contacts these components of thebiosensor. It is a feature of the present invention that the dimensionsof the capillary passageway may vary substantially. In one embodiment,the passageway is a volume that is 1000 μm wide, 100 μm high, and 2000μm long. Other embodiments, and measurement of channels generally, arediscussed in the Analyte Sensors application, referenced above. As thefluid travels along this path, it comes into contact with reagent andelectrodes, as will be described in further detail below.

On substrate 212, contacts 278 are connected via traces 279 toelectrodes 280. These electrodes 280 extend perpendicularly to thelength of the substrate 212, parallel to edge 224 and to each other. Inone preferred embodiment, electrodes 280 are rectangular, with a lengthsufficient to reach across the width of notch 248, a width of at least50 μm, and a separation greater than about 50 μm between nearest pointsthereof. In another preferred embodiment, electrodes 280 are about 100μm wide, with a 100 μm gap. In still other preferred embodiments,electrodes 280 are about 250 μm wide, with a 250 μm gap. Otherconfigurations and dimensions will occur to those skilled in the art,and may be used in the present invention as required or desired giventhe design considerations for a particular strip and system.

Contacts 282 are connected via traces 277 to electrode pair 284.Electrodes 284 each comprise multiple, parallel, elongated rectangles(“fingers”), each extending approximately parallel to edge 224 andperpendicular to the center line of notch 248, reaching at both endsbeyond the width of notch 248. The rectangles connect at one end or theother to trace 274 or 276 in an alternating pattern to form aninterdigitated series of fingers, which will be discussed in furtherdetail below. In various embodiments, each rectangular finger inmicro-electrode pair 284 is between about 5 and about 75 μm in width,with a gap of about 5 to about 75 μm between adjacent fingers. Thefinger widths and the gaps between adjacent fingers are each preferablyconsistent across the width of notch 248.

Turning now to FIG. 3, with continuing reference to FIG. 2, a moremagnified view of the electrode portion of strip 210 in FIG. 2 is shown.As discussed above, electrodes 280 run parallel to edge 224 of strip210, and connect to their conductive traces 270 and 272 at oppositeends, forming electrode pair 266. Their nearest edges 281 are separatedby a distance (“gap”) indicated by reference number 286 that issubstantially constant throughout their length. Similarly,interdigitated fingers 284 form an electrode pair 268, with alternatingfingers connecting to conductive traces 274 and 276.

Turning to FIG. 4, strip 310 shows substrate layer 212, reagent stripe364, notched fluid guide 214, and cover layer 218. In this embodiment,fluid entering capillary notch 348 defined by fluid guide 214 firstencounters macro-electrodes 280. Macro-electrodes 280 are connected viaconductors 379 to contacts 378 at end 368 of strip 310. Electrodes 280are each, for example, about 250 μm in width, and the gap between themis also about 250 μm. Slightly further from strip end 366 is electrodepair 284, which is two electrodes of five fingers each, each finger on aside being connected via a conductor 377 to a contact 382 at strip end368. Each finger in electrode pair 284 is a rectangle about 20 μm inwidth, and each adjacent finger is separated from the next by a gap ofabout 20 μm. Reagent stripe 364 covers electrode pair 280, but notelectrode pair 284.

During a test, when the sample covers electrode pair 280, an AC signalis applied for a period of time to contacts 378. Similarly, for anoverlapping period of time after the sample covers electrode pair 284, aDC signal is applied to contacts 382, and the electrical responsebetween the electrodes in pair 284 is used to estimate the glucoseconcentration in the sample. The response of the sample between thefingers of electrode pair 280 is sensitive to the hematocrit of thesample, which along with a temperature value provided by athermistor-based circuit provides a correction factor for the estimateobtained with electrodes 284. Note that this “correction factor” is notnecessarily a multiplicative or additive factor, but may instead be usedin a formula, in a lookup table, and/or in other ways to correct theestimate based on the temperature and the presence or absence of othermaterials in, or properties of, the sample as will be understood bythose skilled in the art. See, for example, the AC Excitationapplication, which was incorporated above by reference. In thisembodiment, the volume of blood within capillary notch 348 sufficient tocover the measuring electrodes is about 130 nL.

An alternative embodiment is shown in FIG. 5 as strip 410. Substratelayer 212 is traced with two contacts 478, and is partially covered withreagent stripe 464 (over electrodes 480), notched fluid guide 214, andcover layer 218. Contacts 478 are electrically connected via conductivetraces 477 to both a first pair of electrodes 466 and a second pair ofelectrodes 468, one electrode from each pair being connected on eachside to one of contacts 478. Note that in this embodiment the driver andmeter circuitry (not shown) uses a single pair of contacts 478 to driveand measure response from both pairs of electrodes. Note further thatthe relative placements of micro-electrodes 484 and macro-electrodes 480are reversed relative to the embodiment shown in FIG. 4. Themacro-electrodes 480 are again, for example, about 250 μm in width witha gap of about 250 μm between them. Also, each electrode inmicro-electrode pair 466 is made of five fingers that are interdigitatedwith the fingers in the other electrode of the pair. Each finger isagain about 20 μm in width with a gap of about 20 μm between neighboringfingers.

In this embodiment, reagent stripe 464 covers electrode pair 468, butnot electrode pair 466. When the sample covers electrode pair 466, thesystem uses an AC signal through that pair to determine correctionfactors for the analyte measurement. When the sample has coveredelectrode pair 468, an estimate of the analyte concentration is obtainedusing DC excitation methods known in the art, such as U.S. patentapplication Ser. Nos. 09/530,171 and 10/264,891, PCT Application Number(WO) US98/27203, U.S. Pat. No. 5,997,817, and the ElectrochemicalBiosensor Test Strip (reissue) application. With the exemplarydimensions described above, the volume of the capillary cavity is about130 nL.

Turning now to FIG. 6, it can be seen that strip 510 again comprisessubstrate layer 212, reagent stripe 564, notched fluid guide 214, andcover layer 218. In this embodiment, working electrode 581 lies betweentwo counter electrode fingers 580, which are connected by one of theconductors 216 to the same contact. These electrodes 580 and 581 form afirst electrode pair 480, and each of the three macro-electrode fingersin this electrode pair 480 is about 250 μm wide, with a gap of about 250μm on either side of working electrode 581.

Second electrode pair 284 comprises two electrodes of six and sevenfingers each, respectively, the fingers being interdigitated in analternating pattern. Each finger is again about 20 μm wide, with a gapof about 20 μm between adjacent fingers. In this embodiment, the reagentlayer 564 covers both electrode pairs 480 and 284. The macro-electrodepair 480 provides Cottrell-like response, where current is proportionalto the square root of the diffusion coefficient, while themicro-electrode pair 284 provides current that is directly proportionalto the diffusion coefficient. The two responses, taken together, correctfor environmental factors to yield an improved response. The volume ofsample required for measurement in this embodiment is about 200 nL.

Another alternative embodiment is shown in FIG. 7. Strip 610 comprisessubstrate layer 212, reagent stripe 664, notched fluid guide 214, andcover layer 218. As in FIG. 6, the first electrode pair 572 comprisescounter and working macro-electrodes 580 and 581, respectively, eachabout 250 μm wide with a gap of about 250 μm between them. In thisembodiment, however, electrode pair 661 comprises two electrodes ofthree fingers each. Each finger is about 50 μm in width, with a gap ofabout 50 μm between adjacent fingers.

The first electrode pair that the sample reaches (the macro-electrodepair 572) is used to obtain a hematocrit-based measurement using ACexcitation techniques. The second electrode pair (the micro-electrodes661) is used to obtain a measurement that depends on the glucose in andhematocrit of the sample using DC excitation. The reagent stripe 664covers only electrode pair 661, and a sample volume of about 200 nL isrequired to fill the capillary volume in the relevant region. Themeasurements are combined as parameters to a formula based on theelectrode configuration, reagent system, and other factors as wouldoccur to one of skill in the art.

FIG. 8 provides yet another embodiment of the present invention. Strip710 comprises substrate layer 212, reagent stripe 364, notched fluidguide 214, and cover layer 218. In this embodiment, first electrode pair366 comprises two macro-electrodes, each having a single rectangularfinger, while second electrode pair 770 comprises two micro-electrodes,each micro-electrode having five fingers in an interdigitated pattern.The fingers in this embodiment are about 50 μm wide, with a gap of about30 μm between them, and reagent stripe 364 covers second pair 770. Thevolume necessary to cover the electrodes in the relevant portion of thecapillary path is about 170 nL.

Turning now to FIG. 9, strip 810 comprises substrate layer 212, reagentstripe 364, notched fluid guide 214, and cover layer 218. A single pairof contacts 878 is connected via conductors 877 to both first electrodepair 866 and second electrode pair 868. First electrode pair 866comprises two single-finger macro-electrodes 884, while second electrodepair 868 comprises two micro-electrodes 880, each micro-electrode havingfive fingers in an interdigitated pattern. Each electrode in firstelectrode pair 866 is again about 250 μm wide, with a gap of about 250μm between them. First electrode pair 866 is used to obtain a firstmeasurement based on the hematocrit of the sample. Each finger of thesecond pair 868 is about 50 μm wide with a gap of about 30 μm betweenadjacent fingers. When the sample covers second electrode pair 868, a DCsignal is applied to contacts 878. The resulting impedance betweenelectrodes 868 is used to obtain a second measurement based on theconcentration of glucose in and hematocrit of the sample. Thatmeasurement is combined in a formula with the measurement obtainedthrough first electrode pair 866 and a temperature signal from athermistor (not shown) to obtain a corrected glucose concentrationvalue. Reagent stripe 364 covers second electrode pair 868, and therequired volume of sample is again about 170 nL.

FIG. 10 shows another alternative embodiment, strip 1010, whichcomprises substrate layer 212, reagent layer 1064, notched fluid guide214, and cover layer 218. In this embodiment, the first electrode pair1081 encountered by the sample includes working electrode 1071, asingle-finger electrode. First electrode pair 1081 also includes counterelectrode pair 1072, a two-finger electrode, with one finger on eitherside of working electrode 1071. Each finger in first electrode pair 1081is about 250 μm wide, and a gap of about 250 μm separates each counterelectrode finger from the working electrode finger. Each of theelectrodes (i.e., working electrode 1071 and counter electrode 1072) infirst electrode pair 1081 is electrically connected via a conductivetrace 216 to a contact 1067. The system driver connects to contacts 1067to use the first electrode pair to obtain an estimated concentration ofanalyte in the sample.

The second electrode pair 1082 comprises two electrodes of five fingerseach. These fingers are each about 50 μm wide with a separation of about30 μm between them. Each electrode in the second pair connects to aconductive trace 216 to be electrically connected to a contact 1068,which contacts are used to drive and measure for correction factors suchas hematocrit based on the analyte interaction with the second pair ofelectrodes.

The third electrode pair 1083 is also a micro-electrode configuration,with each of the two electrodes in the third pair 1083 having fivefingers interdigitated with the five in the other electrode. Each fingeris again about 50 μm wide, with a gap of about 30 μm between them. Eachelectrode in the third pair 1083 is connected via a conductive trace 216to a contact 1069, and is driven via those contacts to detectsufficiency of the sample volume, based on the electrical responsebetween those electrodes when the sample has reached a sufficient extentthrough the sample cavity 1048. Note that reagent layer 1064 coversupstream electrode pair 1081 in this embodiment. The sample cavity inthis embodiment requires about 220 nL of sample fluid to cover all threeelectrode pairs.

Turning now to FIG. 11, strip 1110 comprises substrate layer 212,reagent stripe 1164, notched fluid guide layer 1114 with notch 1148, andcover layer 1118. The first electrode pair 1170 from the sample end 1166of strip 1110 comprises two electrodes of five fingers each, where eachfinger is about 20 μm wide, and a gap of about 20 μm separate eachadjacent finger. This electrode pair is used for determining theconcentration of interferents such as hematocrit by using AC excitationand impedance measurement techniques. For an example of thesetechniques, see the AC Excitation application, which was incorporatedabove by reference.

The second electrode pair 1171 from sample end 1166 of strip 1110comprises two electrodes of three fingers each. Each finger is about 20μm wide, and a gap of about 20 μm separating adjacent fingers. Thissystem derives a temperature-compensated estimate of glucoseconcentration by applying AC or DC excitation techniques to this secondelectrode pair 1171. The sample volume required to fill the capillarychannel and cover the electrodes in this embodiment is about 69 nL.

Turning now to FIG. 12, strip 1210 comprises substrate 212, reagentstripe 1264, notched fluid guide 1114, and cover layer 1118. The firstelectrode pair 1266 from the sample end 1260 of strip 1210 includes twoelectrodes of five fingers each. This system uses the first pair ofelectrodes 1266 in strip 1210 to obtain one measurement based insubstantial part on detection of interferents for combining with,another measurement, which is obtained using the second electrode pair1268. The second electrode pair from the sample end of strip 1210 iselectrode pair 1268, which includes two electrodes, each having threefingers, and the pair 1268 is covered by reagent layer 1264. The fingersin second electrode pair 1268 are also about 20 μm wide and areseparated by a gap of about 20 μm. This second electrode pair 1268 isused by the system to estimate the concentration of the analyte in thesample. While the first electrode pair 1266 implements AC techniques,the second electrode pair 1268 is driven by an AC or DC signal. Furtherdownstream from the sample end (beyond the second electrode pair 1268)is third electrode 1270, which is a single electrode finger about 20 μmwide, connected via conductor 1274 to contact 1272. The AC signalresponse between this third electrode 1270 and either the firstelectrode pair 1166 or the second electrode pair 1168 provides a samplesufficiency signal for the system. In a variation of this embodiment,third electrode 1270 operates as an electrode in a circuit with secondelectrode pair 1168, for application of various detection andmeasurement techniques known in the art.

FIG. 13 shows strip 1410, which comprises substrate 212, reagent stripe1464, fluid guide 1414 with notch 1448, and cover layer 1418. The firstset of electrodes 1170 from the sample end 1166 of strip 1410 includestwo electrodes, each having five fingers. The fingers in electrodes 1170are each about 20 μm wide, with a gap of about 10 μm separating adjacentinterdigitated fingers.

Second set of electrodes 1171 comprises two electrodes having threefingers each. The fingers of electrodes 1171 are each about 20 μm wide,with a gap of about 10 μm between adjacent, interdigitated fingers.While the first electrode pair 1170 is used by the system to determinethe hematocrit of the sample and calculate a correction factor, anestimate of the glucose concentration is derived from the response ofthe second set of electrodes 1171 in the presence of the sample andreagent. The third pair of electrodes 1471 is two electrodes having twofingers each. In this embodiment, a potential is applied across thethird pair 1471 until the sample reaches the pair, thus changing theimpedance presented between the electrodes. The system can then concludethat the sample has sufficiently covered the first set 1170 and thesecond set 1171 of electrodes for an accurate analysis to be made. Asample volume of about 63 nL is required to cover the three sets ofelectrodes in this exemplary embodiment.

FIG. 14 shows strip 1410, having substrate layer 212, reagent stripe1464, notched fluid guide 1414 (with notch 1448), and cover layer 1418.The first electrode pair 1466 defines a first sensing zone 1476, andcomprises two electrodes of five fingers each. The fingers are about 20μm across, and include a gap of about 20 μm between interdigitatedfingers. This pair 1466 is used to provide a response that reflects thehematocrit of the sample, allowing the system to correct the estimatedconcentration of glucose in the sample as determined by using the secondpair of electrodes 1468. The second pair of electrodes 1468 definessecond sensing zone 1478, and includes two electrodes having threefingers each. The finger sizes and gaps for second electrode pair 1468are the same as those for first electrode pair 1466. Second electrodepair 1468 is used to obtain correction factors for the concentrationestimate obtained by the first electrode pair 1166, and usesAC/impedance measurement techniques.

FIG. 15 shows strip 1510, a variation of the strip in FIG. 11, whereelectrode pairs 1570 and 1571 and the layers covering them would beslightly modified. In particular, electrode pair 1570 comprises aworking electrode having four fingers, each 50 μm wide with a gap widthof 20 μm. The corresponding counter electrode in electrode pair 1570 hasthree fingers, also 50 μm wide. The second electrode pair 1571 comprisesa working electrode having two fingers, each 100 μm wide, and a counterelectrode having a single finger that is also 100 μm wide, with a gapwidth of 20 μm. In this embodiment, reagent 1564 would cover onlyelectrode pair 1571, while coating 1565 would cover electrode pair 1570.Coating 1565 is a perm-selective, size-selective, ion-selective, orother coating that limits the portions or components of the sample thataffect the measurement at electrode pair 1570, as are well known in theart. In variations on this embodiment, three or more electrode pairswould be present, and each electrode pair would be covered with adifferent reagent or other coating, or combination of coatings toprovide a corresponding number of measurements with differentsensitivities, which measurements would be combined to determine thefinal measurement output. In other respects, except constants andfunctions derived from the cell geometry and the selection of coating1565 and reagent 1564, measurement occurs as described in relation toFIG. 11.

Various aspects of the described embodiments can be combined as desiredor necessary, according to the design parameters and preferences for agiven system. For example, there may be a one-to-one correspondencebetween electrodes and contacts on a strip, as shown, for example, inFIG. 4. Alternatively, all electrodes whose fingers are combined on thesame side of a strip may be electrically connected to the same contact,as shown for example, in FIG. 5, providing a many-to-one relationship.

Furthermore, any design discussed herein can accommodate one or more“dose sufficiency” electrodes downstream from those used to analyze thesample, as shown in FIGS. 11 and 14. Such dose sufficiency electrodesmight comprise two or more electrodes, and the associated circuitrycould determine whether the sample has reached those electrodes based onthe impedance presented between them. Alternative embodiments include asingle dose sufficiency electrode, and the meter and driver circuitryuse the impedance between it and a measuring electrode (working orcounter-electrode, estimating or correcting pair) to detect the presenceof the sample fluid in the space between those electrodes.

As previously described, the biosensor may similarly include a dosedetection electrode system that is comparable to the dose sufficiencyelectrode system except that it is located closer to the edge of thetest strip, upstream of the measuring electrodes as the sample entersthe test strip. Such a dose detection electrode system may include asingle electrode that operates in combination with the measurement orother electrodes separately provided. Alternatively, the dose detectionelectrode system may include a pair of electrodes which cooperate withone another to indicate when a sample fluid has bridged the gap betweenthe dose detection electrodes. The dose detection electrodes aretherefore seen to be analogous to the dose sufficiency electrodes interms of operation, but differ as to the location of the electrodes intheir upstream position relative to the measurement electrodes.

In other variations, a thermistor in the system is used to determine thetemperature, which is used along with the hematocrit reading to correctthe glucose estimate. In others, the second pair of electrodes providesa temperature-compensated glucose estimate using techniques known tothose skilled in the art.

In still other variations, the pair of electrodes that the sample firstencounters is a pair of macro-electrodes, while in others, it is amicro-electrode pair. In either case, each electrode comprises 1, 2, 3,4, 5, or more fingers of appropriate dimension, all electricallyconnected both to each other and to a contact for communication with themeter/driver electronics.

Yet further variations use other combinations of measurements to achievedesired results. Generally, these variations apply electrical signals totwo or more electrodes to obtain a corresponding number of responsesignals. Because of the difference in the signal (AC versus DC,spectrum, amplitude, and the like), electrode shape or dimensions,reagent applied to the sample (or possibly the lack of reagent at one ormore electrodes), and/or other differences, the response signals aresensitive to different combinations of analyte concentration andinterferents. In one such example, a first response is correlated withthe hematocrit of the sample, while a second response is correlated witha combination of hematocrit and concentration of glucose in the sample.In another such example, a first response is correlated withtemperature, a second response is correlated with a combination oftemperature and hematocrit, and a third response is correlated with acombination of temperature, hematocrit, and glucose. The resultfunction(s) are likely to vary for each design, but they can bedetermined empirically by those skilled in the art without undueexperimentation.

Those skilled in the art will appreciate that, while the embodimentsherein have been described in terms of combining measurements, or takinga measurement and determining a correction factor, systems according tothe present invention can use any suitable geometry and any appropriatetechnique to obtain and combine the plurality of measurements to achievethe final detection or measurement result. That is, those practicingthis invention may use more or fewer electrodes, and any formula tocombine readings that is suitable in light of the geometries, reagents,and other system design choices made in connection with that design.

As discussed in the Analyte Sensors application, which was incorporatedabove by reference, accurate detection of analytes can be achieved in asmaller-volume strip-based system according to the present invention,without detrimental impact to the connector, than in prior art systems.This allows a smaller sample to suffice for measurement, saving time andhassle for users of the system.

All publications, prior applications, and other documents cited hereinare hereby incorporated by reference in their entirety as if each hadbeen individually incorporated by reference and fully set forth.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiments have been shown and described and thatall changes and modifications that would occur to one skilled in therelevant art are desired to be protected.

What is claimed is:
 1. A test strip, comprising: a first sensor adaptedto test a sample of a bodily fluid in a first measurement region of asample cavity; and a second sensor adapted to test the same samplewithin a second measurement region of the sample cavity; wherein thefirst measurement region and second measurement region total at mostabout 240 nL.
 2. The test strip of claim 1, wherein the totalmeasurement volume is at most about 200 nL.
 3. The test strip of claim1, wherein the total measurement volume is at most about 90 nL.
 4. Thetest strip of claim 1, wherein the total measurement volume is at mostabout 80 nL.
 5. The test strip of claim 1, wherein the total measurementvolume is at most about 65 nL.
 6. The test strip of claim 1, wherein:the first sensor comprises a working electrode and a counter electrode;and the second sensor comprises a single lead.
 7. The test strip ofclaim 1, wherein: the first sensor comprises a first working electrodeand a first counter electrode; and the second sensor comprises a secondworking electrode and a second counter electrode.
 8. The test strip ofclaim 7, wherein the second working electrode and the second counterelectrode are interdigitated arrays of conductive fingers, each fingerbeing substantially rectangular in shape and having a width betweenabout 5 μm and about 50 μm.
 9. A method of measuring concentration of ananalyte in a sample of bodily fluid, comprising: obtaining a firstresponse to an application of a first electrical signal to the sample;obtaining a second response to an application of a second electricalsignal to the sample; and using the first response and the secondresponse to derive a measurement of the concentration of the analyte inthe sample.
 10. The method of claim 9, wherein the obtaining steps areperformed in overlapping periods of time.
 11. The method of claim 9,wherein: the first electrical signal is applied through a firstelectrode pair in electrical contact with the sample; the secondelectrical signal is applied through a second electrode pair inelectrical contact with the sample; and the volume of the sample is lessthan about 240 nL.
 12. The method of claim 11, wherein the volume of thesample is less than about 200 nL.
 13. The method of claim 11, whereinthe volume of the sample is less than about 90 nL.
 14. The method ofclaim 11, wherein the volume of the sample is less than about 80 nL. 15.The method of claim 11, wherein the volume of the sample is less thanabout 65 nL.
 16. The method of claim 9, further comprising: before theobtaining steps, detecting the sufficiency of the volume of the sample.17. The method of claim 16, wherein the detecting is performed using atleast a third electrode.
 18. The method of claim 17, wherein thedetecting is performed using a third electrode pair.